Constant voltage discharge device

ABSTRACT

Circuits and methods for supplying a temporary power supply at a predetermined voltage. A circuit includes a first DC/DC voltage converter that receives an input from a power supply at a first voltage level and generates an output at a second voltage level, higher than the first voltage level. The output is provided to charge a capacitor. A second DC/DC voltage converter has an input connected to the capacitor for drawing power from the capacitor at the second voltage level and generating an output voltage less than the second voltage level. The second DC/DC voltage converter further includes a feedback input that monitors the circuit&#39;s output voltage and activates the second DC/DC voltage converter when the output voltage falls below a predetermined threshold.

RELATED APPLICATION

The present application claims priority from, and is a continuation of,U.S. patent application Ser. No. 10/440,074 filed on May 15, 2003, nowU.S. Pat. No. 7,098,557. The disclosure of the foregoing United StatesPatent Application is specifically incorporated herein by this referencein its entirety and assigned to STMicroelectronics, Inc., Carrollton,Tex., assignee of the present invention.

BACKGROUND

1. Field of the Invention

The present invention relates to electronic circuits, and moreparticularly to an electronic circuit arrangement for supplying aconstant voltage supply to an operating circuit for a period of timeafter power to the operating circuit has been lost.

2. Background

Numerous electronic devices implement a shut-down procedure upon loss ofpower to the device. To execute the shut-down procedures, electricalcircuits in the device must continue to receive power after the devicehas lost power.

By way of example, Telecommunications standard ITU 992.1 requires that aseries of “Dying Gasp” messages be sent by the ADSL Customer Premisesmodem through the data connection to the Central Office modem upon lossof power. Therefore, an ADSL modem requires a source of uninterruptablepower to maintain an existing ADSL modem connection for up to 50milliseconds after input power is lost, in order to complete thetransmission of “Dying Gasp” messages. There is no further need tomaintain power to the modem once the “Dying Gasp” messages are sent.

Accordingly, there remains a need in the art for short-termuninterruptible power supplies for use with electronic devices.

SUMMARY

In an exemplary embodiment, a constant voltage discharge device isprovided. The constant voltage discharge device comprises means forreceiving electrical current at a first voltage level; means forcharging a capacitor to a second voltage level, higher than the firstvoltage level; means for discharging the storage capacitor in responseto a predetermined condition to generate an output current; and meansfor decreasing the output voltage level.

In another embodiment, a method for supplying a constant dischargevoltage is provided. The method comprises receiving an electricalcurrent at a first voltage level; charging a capacitor to a secondvoltage level, higher than the first voltage level; discharging thestorage capacitor in response to a predetermined condition to generatean output current; and decreasing the output voltage level.

In another embodiment, a first circuit for supplying power to a secondcircuit is provided. The first circuit comprises a first DC/DC voltageconverter having an input connected to a power supply for receiving anelectrical current at first voltage and converting the electricalcurrent to a second voltage, greater than the first voltage; a capacitorconnected to an output of the first DC/DC voltage converter for storingenergy at the second voltage level; and a second DC/DC voltage converterhaving an input connected to the capacitor for discharging energy fromthe capacitor and generating an output having a voltage level less thanthe second voltage level.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating an exemplary embodiment of aconstant voltage discharge device;

FIG. 2 is a circuit diagram of an exemplary embodiment of a constantvoltage discharge device; and

FIG. 3 is a schematic depiction of an alternate embodiment of a CVDDthat may be implemented as an 8-pin integrated circuit.

DETAILED DESCRIPTION

The USB bus standard provides a source of up to 500 ma at between 4.75Vand 5.25V for a bus-powered device. At 5 volts, a USB ADSL modem usesabout 400 ma of the available current to provide power to the ADSL modemthrough several DC/DC converters and voltage regulators. Assuming amodem operates normally as long as the input remains at 3.8V or more(constrained by the supply requirements of the 3.3V DC/DC power supply),the minimum amount of reserve energy required to support “Dying Gasp”upon loss of power is: 5 Volts*0.4 Amps*0.05 sec=0.10 Joules. Thefollowing calculations show that a 30,000 uF capacitor can storesufficient energy to support modem operations over the discharge rangefrom 4.75V to 3.8V, but it is not a very efficient use of storage.

$E = {\frac{1}{2}{CV}^{2}}$ E_(total) = 0.5 * 0.03 * (4.75)² = .338  JE_(used) = 0.5 * 0.03 * [(4.75)² − (3.8)²] = .122  JE_(used  %) = 100% * E_(used)/E_(total) = 36%

More of the capacitor's energy store could be used if the dischargevoltage range were increased. This may be achieved by first boosting theinput voltage, but the inefficiencies generated by first boosting theinput then bucking it for all supplies results in a large power losswhich could cause a modem to exceed the 500 ma limit set by the USBstandard.

Alternatively, the voltage may be boosted just on the storage capacitorso that the inefficiencies would be limited to charging and maintainingthe energy reservoir. FIG. 1 is a block diagram illustrating anexemplary embodiment of a constant voltage discharge device. Referringnow to FIG. 1, assume a power source 110 supplies power at a voltage of4.75V to 5.25V. The current passes through a diode 115 to a power supply120 which, in turn, provides power to an electronic circuit 125.Electronic circuit 125 may be embodied as any electronic device. In oneexemplary embodiment, electronic circuit may be an ADSL modem.

In the exemplary embodiment depicted in FIG. 1, a constant voltagedischarge device (CVDD) may be placed in an electrical path between anode 130 connected to the power supply and a storage capacitor 155. TheCVDD may include a first DC/DC voltage converter 140 to step up thevoltage, a diode 145, and a second DC/DC voltage converter 150 to stepdown the voltage.

In operation, input power is passed through the bypass diode 115 to themodem power supply for normal operation of the electronic circuit. Aslong as the supply voltage is above a predetermined threshold, thesecond DC/DC voltage converter 150 (i.e., the step-down converter)remains off while the first DC/DC voltage converter 140 (i.e., thestep-up converter) siphons off a fraction of the input power to boostthe voltage and trickle charge storage capacitor 155 through diode 145.The charging current may be limited to reduce the impact of the circuiton the input source current and charge the capacitor to an acceptablelevel (e.g., 95%) within an acceptable time frame.

When the input power fails or shorts, the first DC/DC voltage converter140 turns off, and the second DC/DC voltage converter 150 switches on,draining the storage capacitor 155 to generate a voltage to the modempower supply below the turn-on threshold of the boost converter 140. Thesecond DC/DC voltage converter 150 ceases to work when the storagecapacitor 155 is discharged below its required input voltage. Boostingthe voltage of storage capacitor 155 enables the use of a much smallercapacitor. For example, compared to the calculations above, calculationsshow that a 25V, 470 uF capacitor can replace a 5V, 30,000 uF capacitoras an equivalent storage device when the useful discharge range ischanged to between 23V and 4.4V:

$E = {\frac{1}{2}{CV}^{2}}$ E_(total) = 0.5 * 0.00047 * (23)² = .124  JE_(used) = 0.5 * 0.00047 * [(23)² − (4.4)²] = .120  JE_(used  %) = 100% * E_(used)/E_(total) = 96%

FIG. 2 is a circuit diagram of an exemplary embodiment of a CVDD 200adapted to boost a 5V input 210 to 22.5 V, which is stored on capacitor240 for subsequent discharge. It will be appreciated that the embodimentdepicted in FIG. 2 is presented by way of example, and not bylimitation. For clarity, only pertinent portions of circuit 200 aredescribed in detail. Referring to FIG. 2, CVDD 200 includes a firstDC/DC voltage converter 220 connected to input 210. The particulardesign of first DC/DC voltage converter 220 is not critical to theinvention. An exemplary DC/DC voltage converter is an LT1930commercially available from Linear Technology, Inc. of Milpitas, Calif.

First DC/DC voltage converter 220 includes an input pin 222 connected toinput line 210 and an output pin 224 connected to line 228. A flybackinductor 214 may be placed in the electrical path between input 222 andoutput 224. First DC/DC voltage converter 220 also includes a feedbackpin 226 that monitors the voltage at a node between resistor 232 andresistor 234 and inhibits first DC/DC voltage converter 220 if thevoltage exceeds a predetermined threshold. An oscillator closes a switchbetween line 224 and ground 270 within chip 220 allowing current to flowfrom 210 to ground through inductor 214. When the oscillator alternatelyopens the path from 224 to ground 270, the voltage across the inductorrises in an attempt to maintain the current. As the voltage at node 228exceeds the voltage at 236 (plus a diode drop) the diode 230 thenconducts current through a limiting resistor 238 to charge the storagecapacitor 240. Diode 230 also blocks reverse current when the internaloscillator of the chip 220 enables the path to ground. The feedback path226 monitors the voltage at node 236 through the resistor pair 232 and234 and inhibits the switching when 22.5 volts is present at node 236.

It will be appreciated that the particular values of resistors 232, 234,238, inductor 214, and storage capacitor 240 are matters of designchoice. In an exemplary embodiment, resistors 234, 238 are 1K resistors,resistor 232 is a 21 K resistor, inductor 214 is a 10 μH inductor, andcapacitor 240 is a 470 μF, 25V capacitor. One of skill in the art couldselect appropriate resistors and capacitors for circuit 200.

CVDD 200 also includes a second DC/DC voltage converter 250 that drainscapacitor 240, stepping the voltage down to a predetermined outputvoltage, e.g., 5 V. The particular design of second DC/DC voltageconverter 250 is not critical to the invention. An exemplary DC/DCvoltage converter is an LT1676 commercially available from LinearTechnology, Inc. of Milpitas, Calif.

In relevant part, second DC/DC voltage converter 250 receives an input252 from line 242 that is connected to capacitor 240 and an output 254that provides the output of circuit 200. A feedback pin 256 monitors theoutput voltage 260 through the resistor divider network consisting ofresistors 262 and 264, and activates the voltage converter 250 if thevoltage at feedback pin 256 drops below a predetermined threshold, e.g.4.7 V.

It will be appreciated that the particular values of resistors 262 and264 are matters of design choice. In an exemplary embodiment, resistor262 is a 33K resistor and resistor 264 is a 12K resistor. One of skillin the art could select appropriate resistors 262, 264 for a desiredactivation voltage.

In operation, assuming 5 volts is present at input 210, the outputvoltage 260 is about 4.7V, i.e., the input voltage less the voltage dropacross diode 212, which is about 0.3V. First DC/DC voltage converter 220activates to generate an increasing output voltage, which chargesstorage capacitor 240 through the current-limiting resistor 238.Excluding any output loading, the maximum input current of the CVDD maydraw less than 10 milliamps. This will charge storage capacitor 240until the voltage drop across resistors 232, 234 exceeds a predeterminedthreshold, in this case 22.5V, at which point the first DC/DC voltageconverter deactivates. At this point, the circuit requires only an idlecurrent, e.g., a couple milliamps as referenced to the input, to powerthe circuit and maintain the energy reservoir.

The second DC/DC voltage converter 250 remains off until the voltage atfeedback pin 256 falls below a predetermined threshold, e.g., 1.2 volts.If the input power is lost or removed or the voltage at node 260 becomestoo low, then the second DC/DC voltage converter 250 activates toprovide the required output voltage by draining storage capacitor 240until the storage capacitor energy is drained below the voltagenecessary to sustain the second DC/DC voltage converter 250. In thisway, circuit 220 provides a temporary uninterruptible power supply onoutput node 260.

One of skill in the art will appreciate that the transition from inputpower to reserve power is smoothed when the voltage output of the secondDC/DC voltage converter 250 is slightly below the voltage at 260 wheninput voltage at node 210 is present. Also, diodes 212 and 230 preventcurrent from flowing back into the input 210, so that even under inputshort circuit conditions, the energy reserve is only sent to the outputload.

It will be apparent to one of skill in the art that circuit 200 providesa means for receiving electrical current at a first voltage level andfor charging a capacitor to a second voltage level, higher than thefirst voltage level, i.e., the first DC/DC converter imposed between apower supply and a storage capacitor. In addition, circuit 200 providesa means for discharging a capacitor in response to a predeterminedcondition, i.e., the second DC/DC converter connected to the storagecapacitor and configured to discharge the capacitor when the voltage atthe output node drops beneath a predetermined threshold.

FIG. 3 is a schematic depiction of an alternate embodiment of a CVDDthat may be implemented as an 8-pin integrated circuit 300. It will beappreciated that the embodiment depicted in FIG. 3 is presented by wayof example, and not by limitation. Again, only pertinent portions ofcircuit 300 will be explained in detail. Referring to FIG. 3, circuit300 includes a boost DC/DC converter comprising a switch 320 and a firstoscillator 330, and a buck DC/DC converter comprising a switch 318 and asecond oscillator 340, and a control module 350. Optionally, circuit 300may also include a power on reset (POR) module for generating a PORsignal.

Circuit 300 is connected to power supply line 302 through line 304. Theswitch 320 is connected between the inductor 312 and ground and iscontrolled by the first oscillator 330 through line 332. The switch 318is connected between the inductor 312 and the storage capacitor 316 andis controlled by the second oscillator 340.

Control module 350 executes the logic to determine whether to enableand/or disable the first oscillator 330 and second oscillator 340. Line352 connects control module to second oscillator 340 and line 354connects control module 350 to oscillator 330. Power on reset (POR) maygenerate a POR signal for one or more second circuits connected tocircuit 300. The control module also initializes oscillators 330 and 340so that switches 318 and 320 default to the open position.

In operation, power is supplied on line 302 at a first voltage level.Switches 320 and 318 remain open when their respective oscillator isinhibited (i.e., not enabled). Control module 350 determines thatcapacitor 316 below its target voltage by comparing the voltage on line334 to an internal reference within control module 350. Additionally, aslong as the reference voltage at Vsense as set by external resistor 322exceeds another internal reference within control module 350, theControl Module generates a signal to activate first oscillator 330 whichalternately closes and opens switch 320. When switch 320 is closed,current flows from line 302 to line 304 through inductor 312 thenthrough switch 320 to ground. When the switch is opened, the path toground is interrupted, and the voltage across the inductor 312 rises inan attempt to maintain the current. As the voltage on line 306 exceedsthe voltage at the storage capacitor 316 (plus a diode drop) the diode314 conducts current through a limiting resistor 308 to charge thestorage capacitor 316 to a second voltage level, higher than the firstvoltage level. When control module 350 senses that the storage capacitorhas either achieved its preset voltage through line 334, or thereference voltage at Vsense falls below its target level, it disablesoscillator 330, leaving switch 320 in the open position.

Control circuit 350 monitors the voltage on line 304, through thevoltage divider connected to Vsense, and if the voltage falls beneath apredetermined threshold as set by the external resistor 322, thencontrol circuit 350 deactivates first oscillator 330 and activatessecond oscillator 340 which alternately closes and opens switch 318 todrain power from capacitor 316. Second oscillator 340 alters its dutycycle in order to maintain the output at a third voltage level to line304 marginally lower than the first voltage level. Inductor 312 acts asboth an energy storage device and low-pass filter in combination withthe external capacitor (322 [attached from line 302 to ground]) tosmooth the output to line 304. During this discharge condition, thevoltage present at Vsense remains below the threshold voltage needed todisable Oscillator 340 and re-enable Oscillator 330. Output ceases whenthe storage capacitor 316 is drained below the level that the oscillator340 can maintain the third output voltage.

Thus, circuits 200 and 300 provide a temporary, uninterruptible powersupply that may be used to provide power to a second circuit in theevent of a failure from the main power supply. Circuits 200 and 300 mayfind application in a wide variety of electronic devices. Exemplaryapplications include: (1) security devices that issue a self-destructsequence upon loss of power, such as in a financial transactionterminal; (2) integrated circuit-based battery death logger or detector;(3) a catastrophic failure alarm; (4) an instantaneous brown-outcompensation device; (5) a processor interrupt and reserve power foremergency data storing; (6) a battery replacement for information backup(i.e., battery+RAM can be replaced by CVDD+flash memory) in areas wherebattery use is discouraged such as medical applications; (7) an air Bagtrigger for total loss of power to automobile; and (8) a remotemonitoring of interrupted power through a broadcast channel.

Although the invention has been described and illustrated with a certaindegree of particularity, it is understood that the present disclosurehas been made only by way of example, and that numerous changes in thecombination and arrangement of parts can be resorted to by those skilledin the art without departing from the spirit and scope of the invention,as hereinafter claimed.

The words “comprise,” “comprising,” “include,” “including,” and“includes” when used in this specification and in the following claimsare intended to specify the presence of stated features, integers,components, or steps, but they do not preclude the presence or additionof one or more other features, integers, components, steps, or groups.

1. A constant voltage discharge device for coupling between a powersource and a power supply of an electronic circuit, the devicecomprising: an input; an output operatively coupled to the input; astorage capacitor; a first DC/DC voltage converter that receives aninput current at a first voltage level and generates an output currentat a second voltage level to charge the storage capacitor; a secondDC/DC voltage converter to discharge the storage capacitor and providean output voltage level at the output below the second voltage level toprovide temporary power to the power supply of the electronic circuitwhen power between the power source and the power supply is interrupted;and a feedback circuit operatively coupled between the second DC/DCvoltage converter and the output to monitor voltage between the powersource and the power supply via the output, such that, when power at theinput fails or shorts, the first DC/DC voltage converter turns off, andthe second DC/DC voltage converter switches on, draining the storagecapacitor to generate the output voltage level.
 2. The constant voltagedischarge device of claim 1, wherein the feedback circuit comprises:first and second resistors operatively coupled between the output and areference voltage; and a feedback path operatively coupled between thefirst and second resistors to provide a sampled voltage from between thefirst and second resistors to the second DC/DC voltage converter.
 3. Theconstant voltage discharge device of claim 1, wherein the second DC/DCvoltage converter provides the output voltage level at the output belowthe first voltage level to provide temporary power to the power supplyof the electronic circuit.
 4. The constant voltage discharge device ofclaim 1, further comprising a second feedback circuit operativelycoupled to the first DC/DC voltage converter to monitor voltage of thestorage capacitor.
 5. The constant voltage discharge device of claim 1,further comprising a current-limiting resistor operatively coupledbetween the first DC/DC voltage converter and the storage capacitor. 6.A circuit for providing temporary power to a power supply of anelectronic circuit when power between a power source and the powersupply is interrupted, the circuit comprising: a storage capacitor; afirst voltage converter that receives an input current at a firstvoltage level and generates an output current at a second voltage levelto charge the storage capacitor; a current-limiting resistor operativelycoupled between the first voltage converter and the storage capacitor; afeedback loop to monitor voltage between the power source and the powersupply; and a second voltage converter to discharge the storagecapacitor and provide an output voltage level below the second voltagelevel to provide temporary power to the power supply of the electroniccircuit; wherein the feedback loop comprises a feedback path providing asampled voltage to the second voltage converter, whereby when power atthe input fails or shorts, the first voltage converter turns off and thesecond voltage converter switches on draining the storage capacitor togenerate the output voltage level.
 7. The constant voltage dischargedevice of claim 6, wherein the feedback loop further comprises first andsecond resistors operatively coupled between an output of the circuitand a reference voltage, wherein the feedback path is operativelycoupled between the first and second resistors, and wherein the sampledvoltage is from between the first and second resistors.
 8. The constantvoltage discharge device of claim 6, wherein the second voltageconverter provides the output voltage level at the output below thefirst voltage level to provide temporary power to the power supply ofthe electronic circuit.
 9. The constant voltage discharge device ofclaim 6, further comprising a second feedback circuit operativelycoupled to the first voltage converter to monitor a voltage of thestorage capacitor.
 10. A method for supplying a temporary power to apower supply of an electronic circuit when power between a power sourceand the power supply is interrupted, the method comprising: receiving aninput current at a first voltage level from the power source andgenerating an output current at a second voltage level with a firstDC/DC voltage converter to charge a storage capacitor; monitoringvoltage between the power source and the power supply; discharging thestorage capacitor and providing an output voltage level to the powersupply below the second voltage level with a second DC/DC voltageconverter to provide temporary power to the power supply of theelectronic circuit when voltage between the power source and the powersupply decreases below a threshold; and when power at the input fails orshorts, turning off the first DC/DC voltage converter, switching on thesecond DC/DC voltage converter, and draining the storage capacitor togenerate the output voltage level.
 11. The method of claim 10, whereinmonitoring comprises providing a feedback circuit including: first andsecond resistors series operatively coupled between the power source anda reference voltage; and a feedback path operatively coupled between thefirst and second resistors to provide a sampled voltage from between thefirst and second resistors to the second DC/DC voltage converter. 12.The method of claim 10, wherein the second DC/DC voltage converterprovides the output voltage level to the power supply below the firstvoltage level to provide temporary power to the power supply of theelectronic circuit.
 13. The method of claim 10, further comprisingoperatively coupling a second feedback circuit to the first DC/DCvoltage converter to monitor a voltage of the storage capacitor.
 14. Themethod of claim 10, further comprising operatively coupling acurrent-limiting resistor between the first DC/DC voltage converter andthe storage capacitor.